net: dsa: Only notify CPU ports of changes to the tag protocol

Previously DSA ports were also included, on the assumption that the
protocol used by the CPU port had to the matched throughout the entire
tree.

As there is not yet any consumer in need of this, drop the call.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>